Orthogonal Frequency Division Multiplexing (OFDM) is a proven access technique for efficient user and data multiplexing in the frequency domain. One example of a system employing OFDM is Long-Term Evolution (LTE). LTE is the next step in cellular Third-Generation (3G) systems, which represents basically an evolution of previous mobile communications standards such as Universal Mobile Telecommunication System (UMTS) and Global System for Mobile Communications (GSM). It is a Third Generation Partnership Project (3GPP) standard that provides throughputs up to 50 Mbps in uplink and up to 100 Mbps in downlink. It uses scalable bandwidth from 1.4 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations. LTE is also expected to improve spectral efficiency in networks, allowing carriers to provide more data and voice services over a given bandwidth. Other wireless standards like WiFi (IEEE 802.11) or WiMAX (IEEE 802.16) also employ OFDM.
In LTE there are several mechanisms by which the terminals inform the Base Station or eNodeB about the radio conditions they are experiencing [1]. The quantity which is defined to measure the instantaneous quality is called Channel Quality Indicator (CQI), and represents a measure of the most suitable Modulation and Coding Scheme (MCS) to be used for a 10% probability of erroneous reception without retransmissions. The parameters CQI may refer to the whole bandwidth or be expressed as a set of values, each corresponding to different frequency subbands. The subbands are comprised of a predetermined number of subcarriers depending on the system bandwidth and the mode of operation. A Frequency Selective Scheduler (FSS) should take advantage of these quantities, which are reported by the User Equipment's (UES) to the eNodeB, in order to assign the available resources so as to maximize the cell capacity and the throughput perceived by each user. Since different mobile terminals will in general observe different frequency-dependent fading profiles, channel-dependent scheduling tends to allocate portions of the overall available bandwidth in a more efficient manner than any arbitrary allocation of bandwidth chunks.
One emerging trend in the field of cellular network architectures is so-called Cloud RAN or Centralized RAN (CRAN). CRAN deployments perform all radio-related procedures of multiple cells at a single central unit, leaving the radio frequency (RF) transmission and reception tasks to the remote radio heads (RRHs). Apart from the cost advantages that can be obtained from hardware centralization (such as reduced operational and maintenance costs), there are additional benefits from centralization of the processing tasks as a result of avoiding inter-cell information exchange. As an example, coordinated scheduling in CRAN has the ability to avoid interferences by simultaneously allocating resources at multiple cells, in such a way that minimal inter-cell interference can be sought. Another example comes from the application of Coordinated Multi-Point (CoMP) techniques, which envisage the transmission/reception at multiple sites in order to reduce interferences and increase cell-edge throughput and overall network capacity. Both techniques require the proper exchange of signaling information and/or data between nodes that will obviously be avoided in CRAN.
Although it is generally perceived that CRAN can bring a lot of new possibilities for RAN deployments, some approaches only aggregate the processing tasks of many sites, thus facilitating inter-cell coordination and resource pooling but not fully exploiting centralized operation. The advantages brought by CRAN can only be exploited if proper scheduling techniques are envisaged. Scheduling should take into account not only the channel characteristics of the users in each of the cells, but also the mutual interferences with other cells so as to maximize the overall capacity. In this scenario, the classical approach of assigning different cell identifiers to each of the cells may not be appropriate, as many cells would eventually have to listen to users' quality reports irrespective of whether they are connected to them or not. Therefore, an alternative approach based on assigning the same cell identity to all the cells may be more effective, as in US patent application US-A1-20140219255. This “super cell” concept ideally avoids handovers and allows the reuse of network resources when there is enough RE isolation between users and sites, thereby increasing capacity without partitioning the network into cells. In addition, even in conditions of significant mutual interference between adjacent users and sites, several COMP-based techniques can be exploited for increased capacity.
The usual approach of aggregating resources in CRAN dismisses new opportunities for more flexible network deployments. Some solutions rely on having the same network topology as distributed RAN has, with different cell identifiers for each of the different sites.
The main drawback of this approach is that users still have to rely on handovers under mobility conditions. A more serious drawback is the lack of flexibility in assigning resources to different sites, being the performance ultimately impaired by interference as well as by the ability of devices to feedback the relevant channel state information in CoMP, which suffers from inherent limitations.
There are initiatives like the one proposed in US-A1-20140219255 where the remote radio heads can be flexibly associated with the same or different cell identities, thereby changing the network configuration in accordance with the central processing unit. However no details are provided in regard to how resources can be efficiently scheduled in the combined space-time-frequency resource grid. The solution provided by US patent application US-A1-20130163539 generalizes the Proportional Fair criterion to a distributed multi-node communications system, however it does not give any actual details on what criteria should be followed in order to perform the nodes association so as to benefit from the coordination capabilities (like CoMP).
More efficient ways to deal with resources scheduling and inter-cell interference are therefore needed in centralized RAN deployments, which motivates the present invention.